Page:The Scientific Monthly vol. 3.djvu/532

 526 THE SCIENTIFIC MONTHLY

a better chance for northern observers to discover comets when the sun is farthest north in June and for southern observers when the sun is farthest south in December. These facts lead to the discovery of comets, prevailingly, which come to perihelion in certain favored re- gions ; that is, in the regions of the sky where the earth is at those times.

It is advantageous at this point to call attention to other sources of lack of homogeneity in comet data.

Prior to the invention of the telescope, three centuries ago, about 400 comets had been made matters of historical record. These were naked-eye objects which forced themselves upon the attention of ob- servers. They were the especially large comets which came close to the earth or to the sim. They were imperfectly observed, and for only a small proportion of them do we know even their approximate orbits.

Since the invention of the telescope, about 450 comets have been discovered, and the half of these have been found in the last fifty years. What we may call the golden age of comet discovery included the two decades, 1888 to 1908, when 100 comets, an average of five per year, were discovered. Four American observers. Swift, Brooks, Bar- nard and Perrine, announced the arrival of thirty-seven of these 100 i comets.

All of the early comets were visible to the naked eye. Only a small fraction of recent comets, perhaps one in four, become bright enough for the unassisted eye to see the head, and perhaps one in eight or ten for the unassisted eye to see the tail. Comet orbits have become in- creasingly accurate, partly because of greater telescopes, which enable these bodies to be more accurately observed and observed through longer arcs of their orbits.

2. Another decisive argument for the theory that comets are at home in the solar system is this : Schiaparelli showed in the early '70's that, owing to the sun's motion through the stellar system, if the comets come from distant interstellar space, a very large proportion of them should move around our sun in hyperbolic orbits, and many of these orbits should be strongly hyperbolic. Schiaparelli's conclusions have been confirmed and extended by several mathematical astronomers, no- tably by Louis Fabry. Fabry concluded: If the sun travels through the stellar system and the comets come to the sun from interstellar space, then the comets should all move in hyperbolas — diflfering from the parabola the more as the velocity of the sun through space is the greater.

What are the facts of observation ? Of 347 comet orbits fairly well determined

(a) 60 are certainly elliptic;

(h) 275 are approximately parabolic;

�� �